The present invention relates to transmitters.
A transmitter generally includes a phase locked loop (PLL) which generates a clock signal. The clock signal or a divided version thereof (in terms of frequency) is locked with a reference clock signal. The frequency range covered by a PLL, among other things, depends on the voltage controlled oscillator (VCO) and the divide factor of the PLL.
One type of VCO used in PLLs is the ring type VCO. A PLL using a ring type VCO is herein referred to as a ring type PLL. The ring type PLL is capable of generating clock signals covering a relatively wide range of frequencies. As a result, such PLLs are sometimes referred to as wide range PLLs. A ring type PLL has the advantage of covering a wide range of frequencies. However, the ring type PLL suffers from relatively high jitter. The jitter worsens with higher clock frequencies. In some cases, the jitter of a clock signal surpasses a tolerance limit for jitter, which makes the clock signal unacceptable.
Another type of VCO is the inductance-capacitance (LC) type VCO. A PLL using an LC type VCO is herein referred to as an LC type PLL. Unlike the ring type PLL, the LC type PLL has relatively low jitter. As a result, such PLLs are sometimes referred to as low jitter PLLs. One disadvantage of an LC type PLL is that it has a relatively narrow frequency range.
It is desirable to have a transmitter that overcomes the above disadvantages.